1. Field of the Invention
The present invention relates to a signal processing device for performing an adjusting process of sound quality such as equalizing with respect to a one-bit digital signal obtained by the sigma-delta (.SIGMA..DELTA.) modulation, a signal recording apparatus for recording the digital signal that is subject to the adjustment of sound quality, and a signal reproducing apparatus for performing the adjustment of sound quality with respect to the one-bit digital signal obtained by the sigma-delta modulation before reproduction.
2. Prior Art
As a method for digitizing an audio signal, there has been known the method for modulating an analog audio signal into a multi-bit audio signal with a quantizing bit length of 16 bits sampled at a sampling frequency of 44.1 KHz.
Recently, the method called the sigma-delta (.SIGMA..DELTA.) modulation is executed for digitizing the audio signal so that the resulting one-bit audio signal is treated as it is.
The one-bit audio signal obtained by the sigma-delta modulation is formatted to have 64 times as great a sampling frequency as the conventional multi-bit audio signal whose sampling frequency is 44.1 KHz and has a quantizing bit as short as as one bit, and has a wide transmittable frequency band. The one-bit audio signal can secure a higher dynamic range than the conventional multi-bit audio signal in an audio band.
Of course, it is necessary to perform a sound effect treatment such as equalizing realized in the multi-bit audio signal even with respect to the one-bit audio signal.
For example, FIG. 1 shows the most fundamental arrangement of the so-called equalizing process for adjusting a frequency characteristic by amplifying and attenuating a specific band of an audible band.
In the signal processing apparatus shown in FIG. 1, an analog audio signal is applied at an input terminal 96 and then is supplied to a sigma-delta modulator 97. The sigma-delta modulator 97 operates to modulate the input analog audio signal into a one-bit digital signal through the sampling frequency 64.times.fs that is 64 times as great as the sampling frequency fs (=44.1 KHz) used in the current compact disk.
The one-bit digital signal that is output from the sigma-delta modulator 97 is supplied to a signal processing circuit 98 for PCM (Pulse Code Modulation). The signal processing circuit 98 performs an equalizing process with respect to the input one-bit digital signal.
The equalized signal that is an output signal of the signal processing circuit 98 is supplied to another sigma-delta modulator 99 in which the equalized signal is modulated back to the one-bit digital signal. Then, the one-bit digital signal is supplied to an output terminal 100.
The signal processing circuit 98 for PCM provides a sampling frequency 64.times.fs to be used for processing the one-bit digital signal. If the signal processing circuit 98 is composed of digital signal processors (DSP), for example, the circuit 98 would need a lot DSPs. Since the audio band has a far lower frequency than the sampling frequency 64.times.fs, the signal processing circuit 98 is required to have greater number of bits for obtaining the desired characteristic. This results in making the overall system greater in scale and cost.